1. Field of the Invention
This invention relates to an improved process for the recovery of uranium values from organic extractants used in solvent extraction processes.
2. Description of the Prior Art
Phosphate rock can contain from 50 to 400 parts per million (p.p.m.) by weight of uranium (expressed as U.sub.3 O.sub.8), depending on its type and origin. A major portion of this uranium becomes solubilized during the acidulation of phosphate rock and ends up as a component of the phosphoric acid. It is estimated that, at the present time, over 4 million pounds of uranium per year are so processed in the United States without being recovered. Organic extractants capable of removing the uranium from the phosphoric acid are presently available. The recovery of uranium values from such organic extractants is an essential step in any overall system for uranium recovery from phosphoric acid by solvent extraction.
The presence of uranium in phosphate rock and in phosphoric acid has been recognized for many years. Consequently, a process as described in "Uranium Recovery from Wet Process Phosphoric Acid" by B. F. Greek, O. W. Allen, and Donald E. Tynan, Industrial and Engineering Chemistry, vol. 49, No. 4, page 608 (1957), was developed and utilized for the recovery of uranium from phosphoric acid produced by treatment of Florida phosphate rock with sulfuric acid. The commercial application of this process was short-lived, however, due primarily to the technical and economic disadvantages which made uranium recovery by this process unattractive as compared to direct uranium production from uranium ores. The major disadvantages of this process included the chemically unstable nature of the extracting reagent, the poor phase separation in the solvent extraction circuit and the expensive pretreatment of the phosphoric acid requiring the use of elemental iron.
Recognizing the disadvantages of the prior art, a research team at the Oak Ridge National Laboratory developed a new solvent extraction system which does not suffer from the shortcomings referred to above. In this system, as reported in "Solvent Extraction of Uranium From Wet-Process Phosphoric Acid", by F. J. Hurst, D. J. Crouse, and K. B. Brown, Oak Ridge National Laboratory, Technical Manuscript 2522, April 1969, the uranium was extracted from phosphoric acid with an organic solution containing a dialkylphosphoric acid and a trialkylphosphine oxide. The uranium-containing organic solution was then subjected to a washing step to remove phosphoric acid and to a stripping step, utilizing an aqueous ammonium hydroxide-ammonium carbonate stripping solution, to concentrate and recover the uranium values. In the stripping operation, the uranium values were transferred from the organic to the aqueous phase. Since the organic solution contained a dialkylphosphoric acid, ammonia values were absorbed into the organic phase from the stripping solution to form the corresponding ammonium salt of this acid. After the stripping operation, the ammonia-carrying organic solvent was returned to the uranium extraction circuit for contact with fresh phosphoric acid. The absorption of the ammonia values by the phosphoric acid resulted in ammonia losses and in undesirable contamination of the phosphoric acid. This new system had certain other distinct disadvantages, namely high ammonia consumption costs resulting from the selection of an aqueous ammonium hydroxide-ammonium carbonate stripping solution, and phosphoric acid losses encountered in the washing step of the solvent extraction circuit.
Most of these drawbacks were corrected by the process described in U.S. Pat. No. 3,737,513, the disclosure of which is included herein by reference. This patent describes the discovery that uranium values can be obtained from an organic extractant containing a dialkylphosphoric acid and a trialkylphosphine oxide by using an acidic aqueous liquor, containing:
(1) a dissolved divalent iron salt, and PA1 (2) a complexing agent selected from the group consisting of phosphoric acid, hydrofluoric acid and mixtures thereof.
The processes heretofore described were also subject to the further drawback that relatively high amounts of iron are carried over with the uranium product. The relatively high amount of iron reports as an impurity in the final U.sub.3 O.sub.8 product unless it is removed. Another drawback in the prior processes was the formation of a precipitate of some kind of phosphatic compounds upon extraction with organic extractant recycled directly from the ammonium carbonate stripping stage. This causes disruption of the extraction operation with losses in overall efficiency of operation, extractant losses and loss of valuable P.sub.2 O.sub.5. The precipitate formed has to be removed to prevent phase separation problems and scaling and clogging of extraction equipment. Also, extractant losses result by entrainment of the valuable extractant around the precipitated solids. No prior art is known which purports to solve these problems.